Jason Malloy has alerted us to an interesting 2003 paper by Sazaki et al. 81 Japanese patients with penis sizes 2 SD below Japanese age norms were genotyped at SRD5A2, which encodes an enzyme that converts testosterone to DHT. Deficient activity of this enzyme results in "various degrees of male pseudohermaphroditism." Three cases showed unusual mutations. One of these, a missense mutation severely reducing enzyme activity, was found in heterozygous condition in 2 of the 100 controls. (One of the special cases was homozygous for the mutation; the other two were heterozyous for the mutation and for additional mutations of the same gene. The three cases also failed to respond to testosterone enanthate treatment for penile lengthening, while responding positively to 5-alpha-DHT so that their penis lengths ended up at "nearly the average of age-matched Japanese controls.") Previous studies have found this mutation homozygous in two Vietnamese brothers with extremely small penises and heterozygous in 5 Chinese males after examination of an ethnically diverse group of 543 males. In the words of the authors, "[t]hese findings imply that R227Q [the missense mutation] may be relatively frequent in Asian populations."

It is perhaps worth pointing out that testosterone levels show a complex and as-yet poorly understood relationship to spatial ability (Pinker, 2002, ch. 18; see also the note here).

The paper is available for free here.